Electrochemical deposition of a metallic layer, usually copper, on a thin resistive substrate—a seed layer—during interconnection formation in ULSI is realized in a plating apparatus consisting of the following components: anodes, power supply, conductive wafer holding device, and an electrolyte cell containing a solution mixture of acid, metal salts and other additives.
In conventional plating, current density across the seed layer is non-uniform, higher at substrate periphery due to a phenomenon called “terminal effect”. This current non-uniformity results in higher plating rate at wafer edge and lower plating rate at wafer center.
The plated film non-uniformity due to plating rate difference between wafer edge and center raises difficulty for the subsequent planarization step in the device process flow. A system of anodes with independent power controls is applied to plating apparatus to overcome the non-uniform plating rate, see U.S. Pat. No. 6,391,166.
Gas bubbles are generated during plating with an inert anode or inert anodes. They may also be introduced from electrolyte feed system and during intervention and routine maintenance done to the apparatus. When these bubbles are in contact with the plating surface of the wafer, voids form in plated film, and the device yield drops. In the most severe case, when a large amount of bubbles are present in the electrolyte, the electrical field can be altered and electrolyte flow in the plating apparatus suffers significant drop due to blockage of the flow path.
De-bubble device based on the idea of buoyancy and natural convection is commonly found in modern plating apparatus. These devices usually do not work as well with small bubbles. Once attached to a surface, small bubbles can hardly be moved by the resultant force from balancing buoyancy force, adhesion force, and drag force under a typical flow rate found in these plating apparatus. These devices consist of a porous layer shaped as an inverted cone with a flat surface. To remove small bubbles in large quantity without altering the flow and electrical fields, a bubble coalescence mechanism, to make small bubbles grow large, and more membrane surface area of the de-bubble device are needed.
As the feature size becomes small, a larger amount of organic additives in the plating solution is required to achieve void-free gapfill. These organic components break down during electroplating process. The resultant break-down products accumulate in the plating bath and degrade gapfill performance. If incorporated into plated film as impurities, they may act as nuclei for void formation, causing device reliability failure.
A higher plating bath bleed and feed rate is usually implemented at high cost for more advanced plating process technologies to ensure chemical freshness.
During plating, fresh active organic species and break-down byproducts exchange rate in the electrolyte near wafer surface, which is mass transport controlled, is not uniform if the flow field in a plating apparatus is not specifically designed. This problem, however, cannot be addressed by a simple rise of bath bleed and feed rate.